1. Field of the Invention
The present invention relates to a process for preparing a large grained UO.sub.2 fuel.
More particularly, the present invention relates to a novel process for preparing a large-grained UO.sub.2 fuel without using any additives.
2. Description of the Prior Art
The conventional process for preparing a large-grained fuel has been performed only by using a pellet of UO.sub.2 fuel for light-water type power reactor and the process is classified broadly into the following three methods:
[Method 1] Extended annealing under reducing conditions at temperatures similar to those used during fuel manufacture (about 1700.degree. C.);
[Method 2] Ultra-high temperature sintering (about 2000.degree. C.); and
[Method 3] Use of additives as Cr.sub.2 O.sub.3, TiO.sub.2, Nb.sub.2 O.sub.5, MgO, etc.
However, in Method 1, a long time (100-200 hours) of sintering is necessary for making the grain diameter grow to a size of 35 .mu.m by sintering at 1,700.degree. C. in a reducing atmosphere.
In Method 2, grains can be produced with a diameter from 30 .mu.m to 35 .mu.m in a few hours. However, an ultra-high temperature furnace technology usable at a temperature of above 2,000.degree. C. becomes a problem.
Thus, at the present time, Methods 1 and 2 cannot be put to economical and therefore, practical use.
In Method 3, a large-grained fuel can be easily obtained in a reasonably short-time by using chemical additives in UO.sub.2 pellet. However, also in a this method, it is necessary to take the various influences of additives on the UO.sub.2 fuel into consideration and its practical use is attended with much difficulty.